1. Field of the Invention
The present invention relates to a mobile telecommunications system and, more particularly, to a mobile telecommunications system advantageously applicable, but not exclusively, to telecommunications with vehicles such as motorvehicles.
2. Description of the Prior Art
Typical of prior art mobile telecommunications systems with which the present invention is particularly concerned is an automobile telephone system which is disclosed in W. R. Young, "Advanced Mobile Phone Service", Bell Syst. Tech. J. Vol. 58, No. 1, pp. 1-278, January 1979. Some of prior art automobile telephone systems share a cellular concept which is such that base stations each covering a limited service zone or cell are arranged two-dimensionally and the service zones of adjoining base stations overlap each other to insure the continuity of a communication.
To terminate an incoming call at a given mobile station, it is necessary to determine the current position of the mobile station. With the prior art cellular automobile telephone system, it has been customary to cause a network to access a plurality of zones by general calling, then detect a response of a mobile station of interest to determine the current position of the mobile station, and then terminate an incoming call at that station.
In a cellular mobile communication system, different frequencies are assigned to the individual zones or cells which adjoin each other so as to avoid interference of electromagnetic waves. To effectively use a limited frequency band available, it is preferable that the zone configuration is fractioned to promote repetitive use of the same frequency. However, fractioning the zone configuration increases the number of times that the frequency should be switched over during movement of a mobile station over a plurality of zones, forcing both of the base stations and the mobile stations to bear an extra load for frequency switchover control. This problem is more pronounced as the moving speed of a mobile body increases. The only implementation available with the prior art cellular system for eliminating this problem is to broaden each zone or to increase the number of frequencies assigned. However, increasing the number of frequencies is extremely difficult in the present servere frequency assignment environment.
An automobile telephone system implemented by the cellular concept is customarily designed to implement voice communications and is therefore not always suitable for services of the kind transmitting a large amount of data at high speed. In land traffic which involves automobiles, for example, services of the kind mentioned include a navigation service which guides automobiles along appropriate routes depending upon the degrees of conjestion, weather and so fourth, and an operation control service which controls the operations of a large number of automobiles collectively with efficiency. In this kind of traffic, therefore, a large amount of data have to be interchanged between on-board units and base stations at high speed. The frequency band of a transmit signal available with a prior art automobile telephone system is limited to the speech band, limiting the applicability of the system.
Further, in the prior art automobile telephone system, in response to every incoming call each base station accesses mobile stations by general calling and, after awaiting a response from a particular mobile station, terminates the call at that mobile station. This results in the need for complicated control over the termination of an incoming call and therefore in a relatively long connection setup time. With the prior art system, it is impossible to locate individual mobile bodies unless the mobile bodies are called up individually. It follows that a transport company or similar user owning a large number of vehicles cannot efficiently supervise the operations of the vehicles.
When a plurality of base stations each being communicatable with a mobile station over a radio link are distributed at spaced locations with a no-wave area intervening between nearby base stations, the base stations will be allowed to share the same frequency as the electromagnetic waves on the radio links. In a mobile communication system having such a configuration, the base station accessible to a mobile station sequentially changes as the mobile station travels and, hence, a communication with a desired mobile station cannot be adequately set up unless the system recognizes the varying position of the mobile station at all times. Moreover, even if the system accurately locates a mobile station at a certain moment, it is not warrantable that it actually stays in that position when a call meant therefore is originated. Especially, when it comes to a system applicable to ordinary roads as distinguished from thruways, a mobile body moves two-dimensionally so that estimating the future movement of a mobile body accurately so as to accomplish efficient call termination control is difficult. Moreover, it is undesirable that the traffic concentrates more on the communication which is adapted to locate mobile stations than on the primary communication traffic of the entire system.
In a mobile telecommunications system, mobile stations constantly move around without remaining in fixed positions so that a plurality of wave propagation paths exist between base stations and mobile stations. The wave propagation paths interfere with each other to cause sharp and noticeable changes in the amount of wave attenuation ascribable to the movement. Hence, in a blind area, a sufficient electromagnetic wave cannot be fed to a mobile station and it sometimes occurs that the path between a land station and a mobile station cannot be set up despite that the land station is kept in connection to a switched telephone network. In this condition, an originating subscriber has to wait until the path to the mobile station has been set up and, in the meantime, the path between the calling terminal and the land station is held only ineffectively. When a predetermined period of time expires before the path between the land station and the mobile station is set up, the originating subscriber is informed of the unable-to-connect condition by, for example, a switched telephone network by voice. Then, the caller has to repetitively call the mobile station until the connection succeeds.